Hydrostatic Axial Piston Machine

ABSTRACT

A hydrostatic axial piston machine defines an adjustable displacement volume and includes a housing, a cylinder drum, a plurality of working pistons, a pivot cradle, and a plain bearing. The working pistons are received in the cylinder drum so as to be axially displaceable, and are supported on a sliding surface of the pivot cradle. The pivot cradle is configured to adjust the displacement volume, and is pivotably mounted on the plain bearing. The plain bearing is fixed with respect to the housing. The hydrostatic axial piston machine is configured to form at least one first hydrostatic relief pressure field between the plain bearing and the pivot cradle.

This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2015 216 333.4, filed on Aug. 26, 2015 in Germany, and to patent application no. DE 10 2016 214 422.7, filed on Aug. 4, 2016 in Germany, the disclosures of which are incorporated herein by reference in their entirety.

The disclosure relates to a hydrostatic axial piston machine.

BACKGROUND

A generic axial piston machine is presented for example in the document DE 10 2012 022 999 A1. Said axial piston machine is of swashplate type of construction with an adjustable displacement volume, and has a cylinder drum which is connected rotationally conjointly to a drive shaft and in which a multiplicity of cylinder bores is formed approximately parallel to the axis of rotation of the drive shaft. In each case one working piston is received in axially displaceable fashion in the cylinder bores. The working pistons are in this case supported on a sliding surface of a pivot cradle which, for the adjustment of the displacement volume, is pivotably mounted on a plain bearing which is fixed with respect to a housing.

To improve the sliding of the pivot cradle on the plain bearing and for relieving the friction surfaces of load, hydrostatic relief pressure fields are provided which can be formed in relief pressure pockets provided for the purpose on the pivot cradle or on the plain bearing. Here, some of the pressure relief pockets have a pressure medium connection, via a pressure medium duct, to the high-pressure side or to the high-pressure port of the axial piston machine, whereas other relief pressure pockets have a pressure medium connection, via a pressure medium duct which extends through the pivot cradle, to a pressure chamber of an actuating cylinder or counterpart cylinder of the adjustment system of the axial piston machine.

A disadvantage of the solution is that the pressure medium connections of the relief pressure pockets to their pressure medium sources are permanent, whereby a permanent leakage flow via the relief pressure fields, and thus a loss of pressure medium and energy, lowers the efficiency of the axial piston machine.

SUMMARY

By contrast to this, it is the object of the disclosure to provide an axial piston machine which exhibits efficiency which can be increased, or is increased, in relation thereto and which exhibits drive unit stability which is increased in relation thereto.

Said object is achieved by way of a hydrostatic axial piston machine according to this disclosure.

Advantageous refinements of the axial piston machine are described in the claims, detailed description, and drawings.

A hydrostatic axial piston machine is of swashplate type of construction and has an adjustable displacement volume. Said axial piston machine has a cylinder drum which is in particular connected rotationally conjointly to a drive shaft and in which a multiplicity of working pistons is received in axially displaceable fashion. Said working pistons are received in particular in cylinder bores which are formed approximately parallel to the axis of rotation of the drive shaft. The working pistons are supported indirectly or directly on a sliding surface of a pivot cradle of the axial piston machine, which pivot cradle, for the adjustment of the displacement volume, is mounted pivotably on a plain bearing which is fixed with respect to a housing, in particular on two symmetrically arranged plain bearings, in particular bearing shells. Here, at least one first hydrostatic relief pressure field can be formed, or is formed, between the plain bearing and the pivot cradle. According to the disclosure, an adjustable first throughflow cross section is provided in a first pressure medium flow path from a first pressure medium source of the axial piston machine to the first hydrostatic pressure relief field.

With the aid of the first adjustable throughflow cross section, the axial piston machine is thus set up for providing the hydrostatic relief in accordance with demand, for example in a manner oriented to particular operating phases or operating points. In this way, leakage losses from the relief pressure field can be minimized, which, in principle, reduces the operating costs of the axial piston machine and can increase the volumetric efficiency of the axial piston machine. In principle, the hydrostatic relief leads to a more precise adjustment of the displacement volume, because, in particular at the start of an adjustment process, less static friction has to be overcome. Furthermore, the pivoting dynamics of the adjustment can be optimized, and control or regulation of the adjustment has proven to be more stable. The stability of the drive unit can, when pivoting is not being performed, be increased by way of increased friction.

In one refinement of the axial piston machine, a second hydrostatic relief pressure field can be formed between the plain bearing and the pivot cradle, in particular for and during the adjustment and/or restoration of the displacement volume. Here, an adjustable second throughflow cross section is provided in a second pressure medium flow path from a second pressure medium source to the second hydrostatic relief pressure field. Through the provision of two or more relief pressure fields, which can thus be supplied with pressure medium independently of one another, the flexibility of the hydrostatic relief of the pivot cradle is increased.

In one refinement, the axial piston machine has an adjustment device which has a hydraulic actuating cylinder for the adjustment of the displacement volume. Here, the first pressure medium source can be formed, in particular is formed, by way of an actuating pressure chamber of the actuating cylinder.

In one refinement, the first adjustable throughflow cross section is formed in the actuating piston.

In one refinement of the axial piston machine, the adjustment device has a hydraulic restoring cylinder for the restoration of the displacement volume, wherein the second pressure medium source can be formed, in particular is formed, by way of a restoring pressure chamber of the restoring cylinder.

In one refinement, the second adjustable throughflow cross section is formed in the restoring piston.

In one refinement, the first pressure relief field can be charged with pressure medium by way of a retraction of the actuating piston into the actuating cylinder, by way of a decrease in size of the actuating pressure chamber. In this way, pressure medium that is otherwise expanded to low pressure can be put to use in the first hydrostatic relief pressure field, whereby pressure medium energy is saved.

In one refinement, the second relief pressure field can be charged with pressure medium by way of a retraction of the restoring piston into the restoring cylinder, by way of a decrease in size of the restoring pressure chamber.

In an alternative refinement, the first pressure medium source is a subset of working pressure chambers which are delimited in the cylinder drum by the working pistons. In particular, the subset is formed by those working chambers which have a pressure medium connection to a high-pressure chamber or high-pressure port, which is fixed with respect to a housing, of the axial piston machine.

In one refinement, the throughflow cross section can be adjusted by way of two parts or components, which are moved relative to one another in a manner dependent on the pivot angle during the change in the displacement volume, of the axial piston machine. Thus, no separate device is provided for the adjustment of the throughflow cross section, with use rather being made of parts of the axial piston machine which are provided in any case and which are moved during the adjustment or restoration. In this way, outlay in terms of control and apparatus is reduced.

In one refinement, an adjustment of the throughflow cross section is coupled to a change, that is to say an adjustment and/or restoration, of the displacement volume, in particular of a pivot angle of the pivot cradle. Here, the coupling may be such that, with increasing displacement volume or pivot angle, the throughflow cross section likewise increases. Alternatively, inverse coupling is also conceivable.

In one refinement, the pressure medium flow path extends through the associated piston (working piston or actuating piston or restoring piston), through a sliding pad of said piston and through the pivot cradle from the respective pressure chamber (working pressure chamber or actuating pressure chamber or restoring pressure chamber) to the respective relief pressure field. In this way, pressure medium flow paths are short, and do not need to be led via, for example, the housing wall. Owing to the shortness of the pressure medium flow paths, a small pressure loss is then possible.

As already mentioned, the adjustable throughflow cross section may be formed by parts of the axial piston machine. In one refinement, it is thus for example the case that the throughflow cross section is delimited by the pivot cradle and by the plain bearing. During an adjustment of the displacement volume, the pivot cradle is pivoted relative to the plain bearing, whereby the corresponding throughflow cross section is changed.

Alternatively or in addition, the adjustable throughflow cross section may be delimited for example by the pairing of parts of pivot cradle/working piston or pivot cradle/sliding pad or working piston/sliding pad. The parts of all of the stated pairings undergo a relative movement with respect to one another during the actuation or restoration, which relative movement can be utilized for the adjustment of the respective throughflow cross section.

In one refinement, the adjustable throughflow cross section is formed by way of an adjustable, in particular separately controllable, throttle device. In this refinement, a facility for the adjustment of the throughflow cross section is thus provided which can be controlled independently of the stated parts of the axial piston machine that are moved during the adjustment or restoration. In this way, the charging of the one or more relief pressure fields with pressure medium is made even more flexible.

It is self-evident that mixed forms are also possible, for example if the first adjustable throughflow cross section is formed by one of the stated pairings of parts and the second adjustable throughflow cross section is formed by way of the adjustable throttle device, or vice versa.

In one refinement, the adjustable throttle device is formed by way of a separately controllable valve or a controllable aperture.

In order to be able to build up the one or more relief pressure fields particularly quickly, and in order to thereby keep in particular static friction between the pivot cradle and the plain bearing and a resulting breakaway torque low, it is provided in one refinement that the valve is in the form of a fast-switching valve (with electromagnetic or piezo actuation). The switching time thereof is in this case in particular shorter or significantly shorter than a switching time of a valve of the axial piston machine by way of which the adjustment device, in particular the actuating cylinder and/or the restoring cylinder, can be or is supplied with pressure medium. It is ensured in this way that the build-up of the one or more relief pressure fields takes place significantly more quickly than the actuation of the pivot cradle for the adjustment or restoration of the displacement volume.

In one refinement, the respective relief pressure field is delimited by at least one relief pressure pocket on the plain bearing, which relief pressure pocket is formed in a bearing surface of the plain bearing. Alternatively or in addition, the respective relief pressure field is delimited by at least one relief pressure pocket on the pivot cradle, which relief pressure pocket is formed in a bearing surface, facing toward the plain bearing, of the pivot cradle. The respective relief pressure pocket may be formed for example by way of a shallow, trough-like recess or a shallow, closed groove.

In one refinement, the relief pressure pocket on the plain bearing is overlapped at least in sections by the relief pressure pocket on the pivot cradle. In this way, a size of the relief pressure field formed by said relief pressure pocket changes during pivoting of the pivot cradle. It is thus possible for a relief force of the relief pressure field to be increased and/or decreased in a manner dependent on the pivot angle.

In one refinement of the axial piston machine, the pivot cradle has a neutral position in which the displacement volume is zero. In the neutral position, the relief pressure pocket on the plain bearing is overlapped at least in sections by the relief pressure pocket on the pivot cradle. Here, the overlap is preferably configured such that, with increasing pivot angle and displacement volume, the relief pressure field increases in size. With increasing displacement volume, the relief force then also increases.

In one refinement, the bearing surface of the pivot cradle extends substantially symmetrically with respect to a neutral plane of the pivot cradle. By contrast, the relief pressure pocket on the pivot cradle extends asymmetrically with respect to said neutral plane. By way of said asymmetry, the resulting relief pressure field can be prepared for a preferred pivot angle range of the axial piston machine.

In one refinement, the pivot cradle has a multiplicity of relief pressure pockets on the pivot cradle which are arranged so as to be spaced apart from one another in a pivoting direction. Alternatively or in addition, the plain bearing has a multiplicity of relief pressure pockets on the plain bearing which are arranged so as to be spaced apart from one another in the pivoting direction. By way of said multiple relief pressure pockets, it is possible for the relief pressure field resulting from them to be extended over a large angle range.

In one refinement, the relief pressure pockets can be supplied with pressure medium via in each case one adjustable throughflow cross section, in particular via in each case one adjustable throttle device, or via a common adjustable throughflow cross section, in particular via a common throttle device. In the former case, the relief pressure field can be built up, in accordance with demand, in a manner flexibly adapted to the pivot angle and the load situation. It is thus possible, for example, for individual or several of the relief pressure pockets to be activated or deactivated in controlled fashion. The latter case constitutes a solution which is simpler in terms of apparatus but which is somewhat less flexible.

In one refinement, the axial piston machine has a control device by way of which the at least one adjustment or the adjustment and the restoration of the pivot cradle can be controlled.

In one refinement, it is furthermore the case that the at least one throttle device is controllable, in particular in a manner dependent on the pivot angle.

In one refinement of the axial piston machine, the control device is designed such that a required change in the displacement volume can be anticipated by said control device from an operating profile, in particular from a profile of one or more operating parameters of the axial piston machine. In one refinement, it is then the case that, in a manner adapted thereto, at least one throughflow cross section of the one or more throttle devices can be controlled, in particular opened and closed in controlled fashion, by way of the control device.

Several exemplary embodiments of a hydrostatic axial piston machine according to the disclosure are illustrated in the drawings. The disclosure will now be discussed in more detail on the basis of the figures of said drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows, in a longitudinal section, a hydrostatic axial piston machine according to a first exemplary embodiment,

FIG. 2 shows a hydraulic circuit diagram of the hydrostatic axial piston machine as per FIG. 1,

FIG. 2a shows, in a schematic illustration, a pressure medium supply of the hydraulic circuit diagram from FIG. 2,

FIG. 3 shows, in a schematic illustration, a pressure medium source, an adjustable throughflow cross section and a relief pressure field of the axial piston machine as per FIGS. 1 and 2,

FIG. 4 shows, in a schematic illustration, a pivot cradle of a hydrostatic axial piston machine as per a second exemplary embodiment,

FIG. 5 shows, in a schematic illustration, a pivot cradle of a hydrostatic axial piston machine as per a third exemplary embodiment,

FIG. 6 shows a working piston with sliding pad of a hydrostatic axial piston machine as per a fourth exemplary embodiment,

FIG. 7 shows a working piston with sliding pad of a hydrostatic axial piston machine as per a fifth exemplary embodiment,

FIG. 8 shows, in a schematic illustration, a plain bearing, a pivot cradle and a working piston with sliding pad of a hydrostatic axial piston machine as per a sixth exemplary embodiment,

FIG. 9 shows, in a schematic illustration, a pivot cradle of a hydrostatic axial piston machine as per a seventh exemplary embodiment,

FIG. 10 shows, in a schematic illustration, the pivot cradle of the hydrostatic axial piston machine as per FIGS. 1 and 2,

FIG. 11 shows, in a schematic illustration, a pivot cradle of a hydrostatic axial piston machine as per an eighth exemplary embodiment, and

FIGS. 12 and 13 show, in a schematic illustration, two exemplary embodiments of relief pressure pockets on the pivot cradle in a developed-view illustration.

DETAILED DESCRIPTION

FIG. 1 shows, in a longitudinal section, a hydrostatic axial piston machine 1. In the exemplary embodiment shown, said hydrostatic axial piston machine is designed as a hydraulic pump and therefore has a dedicated high-pressure port HP and a dedicated low-pressure port LP, which are both formed on a housing cover 2 and are not illustrated in FIG. 1. A drive shaft 4 is driven in rotation about an axis of rotation 3 and drives with it a cylinder drum 6 in which there are provided multiple cylinder bores 8 which are arranged so as to be distributed over the circumference, in which cylinder bores a respective working piston 10 is guided in an axial direction. The working pistons 10 are in each case supported in sliding fashion by way of a sliding pad 11 on a sliding surface 12 of a pivot cradle 13. The latter does not rotate but is adjustable in terms of its inclination by way of an actuating cylinder 14 of an adjustment device. In this way, a pivot angle α, and thus a displacement volume of the axial piston machine 1, can be adjusted between a minimum pivot angle α_(min) and a maximum pivot angle α_(max). At the minimum pivot angle α_(min), the swashplate 12 stands perpendicular to the axis of rotation 3, such that, as the cylinder drum 6 and the working pistons 10 revolve, no stroke movement is generated, and therefore the displacement or delivery volume Vg of the axial piston machine 1 is zero. At the maximum pivot angle α_(max), the sliding surface 12 is turned toward the axis of rotation 3 to a maximum extent such that, as the cylinder drum 6 and the working pistons 10 revolve, the maximum delivery volume Vg_(max) is realized as a result.

The pivot cradle 13 is preloaded in the direction of the maximum pivot angle α_(max) counter to the actuating force of the actuating cylinder 14 by way of a spring 18 of a restoring cylinder 23, which spring is supported on a bushing 16 inserted into the housing cover 2. The actuating cylinder 14 is, in relation to the axis of rotation 3, arranged opposite the restoring cylinder 23. When an actuating pressure chamber 15 of the actuating cylinder 14 is charged with actuating pressure medium, the pivot cradle 13 pivots back to the minimum pivot angle α_(min). Said movement is limited to the pivot angle α_(min)=0° by a stop 20 which is arranged in the interior of the bushing 16. The actuating pressure chamber 15 is delimited firstly by a bushing 17 which is screwed into the housing cover 2 and secondly by an actuating piston 26 which is mounted over the bushing 17. The actuating piston 26 is, by way of a base thereof, supported via a sliding pad on an articulation point (with ball head) which is inserted into the pivot cradle 13 at an edge side spaced apart from the pivot axis. The bushing 16, the stop 20 and a restoring piston 22 that protrudes into the bushing 16 delimit a restoring pressure chamber 24. The restoring piston 22 is coupled to the pivot cradle 13 by way of a sliding pad and an articulation point (with ball head) spaced apart from the pivot axis.

The pivot cradle 13 has, parallel to the pivot axis and symmetrically with respect to the section plane illustrated in FIG. 1, two partially cylindrical bearing segments 28 which are in each case pivotably received in a plain bearing 30 which is embedded in a bearing block. In FIG. 1, the bearing segment 28 arranged behind the section plane and the associated plain bearing 30 are illustrated by dashed lines because they are concealed by the pivot cradle 13. Multiple relief pressure pockets 232 are formed into the plain bearing 30 so as to be distributed circumferentially in the pivoting direction, which relief pressure pockets can be supplied with pressure medium by way of a pressure medium source of the axial piston machine 1. In this way, a relief pressure field can be built up between the plain bearing 30 and the pivot cradle 13, more specifically the bearing segment 28 thereof.

Before the embodiment of the pressure medium charging of said relief pressure field is discussed with reference to FIGS. 2 and 2 a, it is the intention to illustrate which basic possibilities for the pressure medium supply to relief pressure pockets, and for the configuration thereof, are proposed.

The illustrations in the following FIGS. 3 to 13 are highly schematic.

FIG. 3 shows the principle by which one or the multiple relief pressure pockets 32 are supplied with pressure medium in order to build up the relief pressure field between the bearing segment 28 of the pivot cradle 13 and the plain bearing 30. For this purpose, a pressure medium source 36 is provided which has a pressure medium connection to the relief pressure pockets 32 via a pressure medium flow path 34. Here, the pressure medium connection is controlled by way of an adjustable throughflow cross section, in particular that of a throttle device, arranged in the pressure medium flow path 34. Here, the throughflow cross section may be formed for example by parts which are moved relative to one another in any case during the pivoting of the pivot cradle, for example the pivot cradle and the plain bearing. Alternatively, the throughflow cross section may be formed by a separately controllable throttle device, for example a valve.

In FIG. 4, the supply of pressure medium to a relief pressure pocket 32 on the pivot cradle, which relief pressure pocket is formed into the sliding surface of the bearing element 28, is realized via those working chambers 9 of the axial piston machine 1 which are charged with high pressure. The figure shows the working piston 10 which is supported on the sliding surface 12 in the pivot cradle. Here, the working piston is supported directly on the sliding surface 12 and is suitably sealed to said sliding surface such that the leakage in the contact region between working piston 10 and pivot cradle 13 is minimal. The relief pressure pocket 32 is connected to the working chamber 9 via the pressure medium flow path 34 which extends through the pivot cradle 13. Here, the pressure medium flow path 34 also extends through the working piston 10. Here, in the pressure medium flow path 34, the adjustable throughflow cross section is in the form of a separately actuable throttle device 38. The relief pressure pocket 32 is of asymmetrical design in relation to a neutral plane 40 of the pivot cradle. Here, a relatively large section of the relief pressure pocket 32 extends counter to the pivoting direction for an enlargement of the displacement volume. The relatively small section extends in said direction.

FIG. 5 shows a further exemplary embodiment in which the separately actuable throttle device in the working piston 10 is dispensed with and, instead, the adjustable throughflow cross section is provided at the support point of the working piston 10 on the sliding surface 12. The relative movement of the pivot cradle 13 relative to the working piston 10 supported thereon gives rise to an adjustment of the throughflow cross section which is defined by way of the opening-out points of the pressure medium flow path on the working piston and on the pivot cradle. Here, to be able to cover the pivot angle range of the pivot cradle, the opening-out point of the pressure medium flow path 34 in the sliding surface 12 is designed to be slightly larger, such that the displacement of the opening-out point of the working piston 10 which occurs during the pivoting movement relative to the opening-out point in the sliding surface 12 can be covered.

FIG. 6 shows a further exemplary embodiment, in which the adjustable throughflow cross section 38 is formed by geometries of the sliding pad 11 and of the working piston 10. Here, the pressure medium flow path 34 extends through the working piston 10 and the sliding pad 11. Here, a first section 34 a opens out at an end side in a ball head of the working piston 10. A second section 34 b extends through the sliding pad 11 from its side in contact with the sliding surface to a spherical recess in which the ball head is received.

FIG. 7 diagrammatically shows the control principle as has already been discussed in FIG. 4, in the case of which the adjustable throughflow cross section is arranged as an adjustable throttle device 38 in the working piston 10. The sliding pad 11 is also illustrated in this case.

FIG. 8 shows an exemplary embodiment in which the feed flow of pressure medium into the pivot cradle 13 is realized via a working piston 10 with sliding pad 11. The adjustable throughflow cross section is in this case situated between the sliding pad 11 and the pivot cradle 13.

FIG. 9 shows an exemplary embodiment in which, in addition to the relief pressure pocket 32 on the pivot cradle, a relief pressure pocket 232 on the plain bearing is provided. In the neutral position, the two relief pressure pockets 32, 232 overlap. If the pivot cradle is then pivoted in the direction of an enlargement of the displacement volume, that is to say in the direction α⁺, the relief pressure field formed by the two relief pressure pockets 32, 232 is enlarged. Accordingly, the pivot cradle is hydrostatically relieved to an increasing extent with increasing pivot angle α.

FIG. 10 shows a further exemplary embodiment with multiple relief pressure pockets 232 on the plain bearing which are arranged so as to be spaced apart from one another in the pivoting direction α⁺ and which are fluidically connected via the pressure medium flow path 34 to the pressure medium source 36. Here, each relief pressure pocket 232 is connected via a parallel branch of the pressure medium flow path 34. The relief pressure pockets 232, which are thus supplied with pressure medium in parallel, can be supplied with pressure medium via a common throttle device 38 which is designed as a separately actuable valve.

As an alternative to this, the supply of pressure medium to the relief pressure pockets 232 may be configured as per FIG. 11. Here, each parallel branch of the pressure medium flow path 34 has a dedicated throttle device 38, which throttle devices are in each case separately actuable. It would also be conceivable for fewer throttle devices 38 than relief pressure pockets 232 to be provided, that is to say for one throttle device to be assigned to multiple relief pressure pockets. In this way, it is possible, in a manner dependent on the pivot angle, for the individual relief pressure pockets 232 to be supplied with pressure medium or to be shut off from the pressure medium supply of the pressure medium source 36. Alternatively or in addition, each individual one of the relief pressure pockets 232 may be supplied with pressure medium with a different throttling cross section of the respective throttle device 38. In this way, the resulting relief pressure field can be adapted in a highly effective manner to the pivot angle α and to the forces locally occurring here in the plain bearing 30.

FIGS. 12 and 13 show relief pressure pockets 32 on the pivot cradle in a schematic illustration. The figures each show the relief pressure pocket 32 and an associated opening-out point of the pressure medium flow path 34 that provides a supply to said relief pressure pocket. The relief pressure pockets 32 are arranged symmetrically with respect to the neutral plane 40. In FIG. 12, on each bearing segment 28 of the pivot cradle 13, two relief pressure pockets 32 are arranged on each side of the neutral plane 40. Opening-out points of the pressure medium flow path 34 into the respective relief pressure pocket 32 are arranged diagonally with respect to one another, in each case in a corner of the relief pressure pockets 32 which, in principle, are rectangular. The relief pressure pockets 32 as per FIG. 12 are supplied with pressure medium via the actuating pressure chamber 15 and via the restoring pressure chamber 24 of the hydraulic pump as per FIG. 1. Here, the relief pressure pockets 32 indicated by A are charged with pressure medium via the restoring pressure chamber 24, and the relief pressure pockets 32 indicated by B are charged with pressure medium via the actuating pressure chamber 15.

By contrast to the exemplary embodiment as per FIG. 12, FIG. 13 shows relief pressure pockets 32 which extend across the neutral plane 40 and symmetrically with respect thereto. In this case, too, the relief pressure pockets 32 indicated by A are supplied with pressure medium from the restoring pressure chamber 24, and the relief pressure pockets indicated by B are supplied with pressure medium from the actuating pressure chamber 15. Compared with the relief pressure pockets as per FIG. 12, the relief pressure pockets as per FIG. 13 are of narrower form but have the same area.

FIG. 2 shows a hydraulic circuit diagram of the axial piston machine 1 for the purposes of illustrating the pressure medium supply of the relief pressure pockets 232 as per FIG. 10, which pressure medium supply is, for improved clarity in FIG. 2, shown on a smaller scale in FIG. 2 than in FIG. 2a . The axial piston machine 1 has a high-pressure line 42 and a low-pressure line 44. Here, in the pump operating mode, said axial piston machine delivers pressure medium from the low-pressure line 44 to the high-pressure line 42. The illustration in FIG. 2 illustrates the actuating cylinder 14, which acts on the pivot cradle 13, and the restoring cylinder 23, which likewise acts on the pivot cradle 13. The axial piston machine 1 has an electromagnetically actuable 3/3 proportional directional valve 46, by way of the first end position 46 a of which, when the associated electromagnet a is energized, a pressure medium connection is established between the high-pressure line 42 and the actuating pressure chamber 15. By way of an end position 46 b which is assumed when the electromagnet B of the valve 46 is energized, the actuating pressure chamber 15 is connected to the tank T. In the first end position 46 a, the actuating piston 26 is deployed, and thus the pivot angle α is decreased, whereas in the second end position 46 b, the actuating piston 26 is retracted, and the pivot angle α is increased.

The restoring pressure chamber 24 of the restoring cylinder 23 can be placed in pressure medium connection with the high-pressure line 42 by way of a 2/2 directional switching valve 52. Said valve is preloaded into a closed position by way of a spring and can, when electromagnetically actuated, be adjusted into a throughflow position such that the stated pressure medium connection is opened up.

By way of an adjustable throttle device 38 which is designed as a 2/2 directional switching valve and which is electromagnetically actuable, it is possible, as per FIGS. 2 and 2 a, for the actuating pressure chamber 15 to be placed in pressure medium connection with the relief pressure pockets 232 arranged on the plain bearing. In FIG. 2, an optional adjustable throttle device (38, dashed lines) is illustrated by way of which the restoring pressure chamber 24 can be placed in pressure medium connection with optional relief pressure pockets (32, dashed lines) on the pivot cradle.

The axial piston machine 1 also has, for the control of its displacement volume and of the relief pressure field, a control device ECU. For explanation of the pressure medium supply of the relief pressure pockets 232, it shall firstly be assumed that the axial piston machine 1 is being operated in a steady state with a static pivot angle α. Accordingly, it is presently the case that no adjustment of the pivot angle is taking place. At this time, the 2/2 directional switching valve 52 is electromagnetically actuated, such that the restoring pressure chamber 24 is supplied with high pressure from the high-pressure line 42. The 3/3 proportional directional valve 46 is adjusted in the direction of the end position 46 a by way of the control device ECU and the energization of the electromagnet a, such that the high-pressure line 42 has a throttled pressure medium connection to the actuating pressure chamber 15. The forces of the spring 18 acting on the pivot cradle 13, of the high pressure acting on the restoring piston 22 and of the pressure in the actuating pressure chamber 15, which acts on the actuating piston 26, hold the pivot cradle 13 in equilibrium, that is to say with a constant pivot angle α. Since the pivot cradle 13 is presently not being adjusted, the adjustable throttle device 38 is not electromagnetically actuated, such that the spring forces the throttle device 38 into its blocking position. Correspondingly, the relief pressure pocket 232 is not charged with pressure medium, and the relief pressure field does not exist, whereby it is also the case that leakage is prevented.

It is now sought to realize a decrease of the pivot angle α, which means that pressure medium is to be supplied to the actuating pressure chamber 15. Before this takes place, the electromagnet of the throttle device 38 is energized by way of the control device ECU such that the throughflow position of said throttle device connects the actuating pressure chamber 15 to the relief pressure pocket 232. Correspondingly, the relief pressure field builds up between the pivot cradle 13 and the plain bearing 30. Subsequently to this, the magnet a of the 3/3 proportional directional valve is actuated, such that the valve body thereof is displaced to a greater extent in the direction of the first end position 46 a, and the throughflow cross section is increased in size. In order to provide the stated time gap between the build-up of the relief pressure field and the adjustment, the throttle device 38 may for example be designed as a fast-switching valve with a significantly shorter switching time than the 3/3 proportional directional valve. With the supply of pressure medium into the actuating pressure chamber 15, the actuating piston 26 is displaced counter to the hydrostatic force and the spring force of the restoring cylinder 23 until a new force equilibrium on the pivot cradle 13 has been established and the adjustment has thus come to an end. At this time, it is also the case that the energization of the electromagnet of the throttle device 38 is deactivated again, such that the relief pressure pocket 232 is separated from its pressure medium supply and the relief pressure field breaks down, such that further leakage via said relief pressure field is prevented.

During the increase of the pivot angle α, the pressure medium supply is controlled as follows: firstly, before the adjustment, the electromagnet of the throttle device 38 is energized again, such that the pressure medium connection of the actuating pressure chamber 15 to the relief pressure pocket 232 is established and the relief pressure field can be built up. Then, by way of a deactivation of the energization of both electromagnets a and b of the 3/3 proportional directional valve 46 by way of the control device ECU, said valve 46 is adjusted into a spring-centered central blocking position 46 c in which the actuating pressure chamber 15 is separated from the high-pressure line 42 and is simultaneously placed in throttled pressure medium connection with the tank T via a bypass line 48 and an aperture 50 arranged therein. Accordingly, with the actuating piston 26 retracted, pressure medium can be discharged from the actuating pressure chamber 15 to the tank T, and at the same time, an adequately high pressure for forming the relief pressure field can be maintained in the actuating pressure chamber 15. The pivot cradle 13 performs an adjustment in the direction of the greater pivot angle α until force equilibrium on the pivot cradle 13 has been established again. Then, the energization of the electromagnet of the throttle device 38 is deactivated, whereby the pressure medium connection of the actuating pressure chamber 15 to the relief pressure pocket 232 is shut off, and the relief pressure field breaks down.

The disclosure discloses a hydrostatic axial piston machine of swashplate type of construction, for which a hydrostatic relief pressure field can be built up between a pivot cradle and a plain bearing. Here, for the supply of pressure medium to the relief pressure field in accordance with demand, an adjustable throughflow cross section is provided. Said adjustable throughflow cross section can be varied in terms of its cross section in particular in a manner dependent on an adjustment or restoration of the displacement volume.

LIST OF REFERENCE DESIGNATIONS

1 Hydrostatic axial piston machine

2 Housing cover

3 Axis of rotation

4 Drive shaft

6 Cylinder drum

8 Cylinder bore

9 Working pressure chamber

10 Working piston

11 Sliding pad

12 Sliding surface

13 Pivot cradle

14 Actuating cylinder

15 Actuating pressure chamber

16, 17 Bushing

18 Spring

20 Stop

22 Restoring piston

23 Restoring cylinder

24 Restoring pressure chamber

26 Actuating piston

28 Bearing segment

30 Plain bearing

32; 232 Relief pressure pocket

34 Pressure medium flow path

36 Pressure medium source

38 Throttle device

39 Bearing surface

40 Neutral plane

42 High-pressure line

44 Low-pressure line

46 3/3 proportional directional valve

46 a, 46 b End position

46 c Blocking position

48 Bypass line

50 Aperture

ECU Control device

α Pivot angle

α_(min) Minimum pivot angle

α_(max) Maximum pivot angle

Vg Displacement volume

Vg_(max) Maximum delivery volume 

what is claimed is:
 1. A hydrostatic axial piston machine, defining an adjustable displacement volume, and comprising: a housing; a cylinder drum; a plain bearing fixed to the housing; and a pivot cradle that is pivotably mounted on the plain bearing to enable formation of at least one first hydrostatic relief pressure field between the plain bearing and the pivot cradle, the pivot cradle defining a sliding surface and being configured to adjust the displacement volume; a plurality of working pistons received in the cylinder drum so as to be axially displaceable, and supported on the sliding surface of the pivot cradle; and a first pressure medium source; the hydrostatic axial piston machine further defining a first pressure medium flow path from the first pressure medium source to the first hydrostatic relief pressure field, the first pressure medium flow path including an adjustable throughflow cross section.
 2. The hydrostatic axial piston machine of claim 1, further comprising: a second pressure medium source, wherein: the mounting of the pivot cradle on the plain bearing further enables formation of a second hydrostatic pressure relief field between the plain bearing and the pivot cradle; and the hydrostatic axial piston machine further defines a second pressure medium flow path from the second pressure medium source to the second hydrostatic relief pressure field, the second pressure medium flow path including a second adjustable throughflow cross section.
 3. The hydrostatic axial piston machine of claim 1, further comprising: an adjustment device that includes a hydraulic actuating cylinder configured to adjust the displacement volume; wherein the hydraulic actuating cylinder defines an actuating pressure chamber that enables formation of the first pressure medium source.
 4. The hydrostatic axial piston machine of claim 3, wherein: the adjustment device further includes a hydraulic restoring cylinder configured to restore the displacement volume; and the hydraulic restoring cylinder defines a restoring pressure chamber that enables formation of the second pressure medium source.
 5. The hydrostatic axial piston machine of claim 1, wherein: the working pistons delimit a plurality of working chambers in the cylinder drum; and the first pressure medium source includes a pressure medium connection to a subset of the plurality of working chambers in the cylinder drum.
 6. The hydrostatic axial piston machine of claim 2, wherein the hydrostatic axial piston machine is configured such that an adjustment of at least one of the first adjustable throughflow cross section and the second adjustable throughflow cross section is coupled to a change in the displacement volume.
 7. The hydrostatic axial piston machine of claim 3, wherein: the working pistons delimit at least one working chamber in the cylinder drum; and the first pressure medium flow path extends through the pivot cradle from at least one of the at least one working chamber and the actuating pressure chamber to the first hydrostatic relief pressure field.
 8. The hydrostatic axial piston machine of claim 1, wherein: the sliding surface of the pivot cradle and at least one of the working pistons together delimit the first adjustable throughflow cross section; at least one working piston includes a sliding pad, and the sliding surface of the pivot cradle and the sliding pad together delimit the first adjustable throughflow cross section; or at least one working piston includes a sliding pad, and the at least one working piston and the sliding pad together delimit the first adjustable throughflow cross section.
 9. The hydrostatic axial piston machine of claim 1, further comprising at least one of: a first throttle device that forms the first adjustable throughflow cross section; and a second throttle device that forms the second adjustable throughflow cross section.
 10. The hydrostatic axial piston machine of claim 9, wherein at least one of the first throttle device and that second throttle device respectively includes a controllable valve or a controllable aperture.
 11. The hydrostatic axial piston machine of claim 1, wherein at least one of: the plain bearing defines a first bearing surface that includes at least one first relief pressure pocket, the at least one first relief pressure pocket delimiting the first hydrostatic relief pressure field; and the pivot cradle defines a second bearing surface that faces toward the plain bearing and that includes at least one second relief pressure pocket, the at least one second relief pressure pocket delimiting the first hydrostatic relief pressure field.
 12. The hydrostatic axial piston machine of claim 11, wherein in a neutral position of the pivot cradle: the displacement volume is zero; and the first relief pressure pocket overlaps the second relief pressure pocket, at least in sections.
 13. The hydrostatic axial piston machine of claim 11, wherein: the pivot cradle is configured to pivot about a pivot axis that lies on a neutral plane of the pivot cradle; the second bearing surface of the pivot cradle extends substantially symmetrically relative to the neutral plane; and the second relief pressure pocket extends asymmetrically relative to the neutral plane.
 14. The hydrostatic axial piston machine of claim 1, wherein at least one of the plain bearing and the pivot cradle defines a plurality of relief pressure pockets spaced apart from each other along a pivoting direction.
 15. The hydrostatic axial piston machine of claim 14, wherein the first adjustable throughflow cross section is configured to supply at least one of the plurality of relief pressure pockets with pressure medium.
 16. The hydrostatic axial piston machine of claim 6, wherein the hydrostatic axial piston machine is configured such that an adjustment of at least one of the first adjustable throughflow cross section and the second adjustable throughflow cross section is coupled to a change in a pivot angle of the pivot cradle. 